The present invention relates to process control utilizing optical markings in general and to optical markings used to detect displacement in particular.
In many experimental and industrial processes, imaging of marks placed on an object are used to detect displacement of the object or portions thereof. Determination of displacement between two marks on an object can be used to determine strain, vibrational displacement, orientation, bending, and displacement and its derivatives: velocity and acceleration.
Marking an object generally involves painting or attaching visible marks to the object. In manufacture, the marks may add expense both in applying and removing. Further, some products such as paper are rendered useless by marking. Moreover, in experimental applications it can be difficult to accurately apply marks in a regular pattern. In some experimental situations the markings may themselves interfere with the experiment, particularly with biological systems.
In many manufacturing processes a web or sheet of material is handled and processed through multiple steps. Oftentimes the web or sheet is oriented, distorted, bent, formed, or cut at each step. Process control of the often rapidly moving sheet can demand that witness marks be painted on the sheet being processed to allow high speed imaging of the sheet to detect motion, orientation and plastic or elastic strain in the sheet.
Examples of processes where monitoring a rapidly moving web of material is important are papermaking and secondary papermaking process steps such as coating and slitting the paper. Because paper is featureless, markings must be placed on the web if process controls utilizing image processing are to be used. Many techniques are now available for integrating controls with computer imaging systems, but such systems require visually distinct marks on the object being imaged. Marking paper, however, destroys its economic value.
What is needed is a method of marking an organism, object, web or sheet of material which is only temporary in nature but which allows the marks to be readily imaged
The method of this invention involves selectively heating portions of a biological organism, object, structure, sheet or moving web with, for example infrared radiation, thereby locally heating and marking the object with a thermal spot. The thermal spot is detectable by an infrared camera and allows tracking the motion of the heated spot over time and extracting kinematic information about the marked object. This technique is most effective with materials which have low thermal conductivity and high emissivity. But most objects have an emissivity and thermal conductivity which allows persistence of a mark for a useful period of time. A paper web is an example of a material where a temperature rise of a few degrees will typically remain visible for many seconds to a minute. A thermal camera can detect a fraction of a decree temperature difference and, for temperatures near room temperature, cooling by radiation dissipates energy relatively slowly. For paper manufacture or processing, the paper web is often processed at 50 to 100 feet per second, so a thermal mark made on the paper""s surface will persist for many hundreds of feet.
The simplest marking is a witness mark which allows a clocking of web speed throughout a set of processes. Two marks allow strain to be detected. More complicated patterns include a grid of dots or lines which would allow complete determination of strain in the web.
Grid lines also allow precise determination of orientation between two manufacturing stations which interact with an object. The sheet or object can be marked when the sheet is in a known orientation so that a correlation between the pattern marked by the thermal image and the object is known, or a grid can be written/projected onto an object when its orientation is unknown and its orientation can be determined later when the object is in a known reference frame.
A grid of lines or other regular pattern of lines or circles can be used to form moire patterns between a first image and a second image in which the original image is displaced or rotated. Extreme precision is possible utilizing moire techniques.
In plane vibration can be monitored on structures by tracking motion of thermal spots. Three dimensional information can be obtained by utilizing two or more thermal imaging devices to obtain stereo images. For monitoring vibrational scale movements sub-pixel level changes must be detected. Detection of sub-pixel image shifting is facilitated by knowledge of the thermal gradient along the edges of the thermal spot being detected. This determination of thermal gradient can be by correct observation with a thermal imaging device or with greater sensitivity and simplicity the image may be artificially dithered by motion of an optical element at a known frequency. Analysis of the image data then produces a value for thermal gradient which can be used to detect sub-pixel motion of the image of a thermal spot.
Biomechanical data can be gathered by placing thermal marks on a living person or organ and monitoring displacement of muscles, limbs, joints. Many biological systems are essentially elastic membranes. Three dimensional imaging of the motion of such biological membranes is possible by imaging thermal spots with two or more thermal imaging devices.
It is an object of the present invention to provide a method of temporarily marking objects.
It is another object of the present invention to facilitate process control of moving webs of material.
It is a further object of the present invention to allow the orientation of an object to be determined by computer imaging techniques without permanently marking the object.
It is a yet another object of the present invention to provide a method of determining bending, stretching, or deformation of an object without permanently marking the object.
It is a yet further object of the present invention to provide a method of temporarily marking selected areas of an object for later processing.
It is a still further object of the present invention to provide a method of detecting very small motion of thermal spots by determining the thermal gradient at the periphery of said thermal spots.
It is still another object of the present invention to provide a method of imaging elastic membrane motion in three dimensions.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.